Offshore structures are in constant need of protection from the corrosive environment of seawater. The useful life of offshore steel structures such as oil well drilling and production platforms and piping systems and particularly the tensioned, high strength steel mooring and riser elements of a tension leg platform system are severely limited by the corrosive environment of the sea. Conventional protection against such damage adds considerable complications and weight to offshore structures.
Cathodic protection by either sacrificial anodes or impressed current is generally effective in preventing corrosion on fully submerged portions of an offshore structure. In some offshore locations, such as the North Sea, oxygen content is relatively high even in water depths up to 1,000 feet. As a consequence, oxidative corrosion is very severe and can occur at these depths.
Installation and maintenance of sacrificial anodes adds greatly to the weight and expense of an offshore structure. This is particularly true with respect to a tension leg platform. In a tension leg platform, high strength, thick walled steel tubulars are constantly maintained in tension between their anchor points on the ocean floor and a floating structure whose buoyancy is constantly in excess of its operating weight. The use of high strength steel in the TLP for fabricating the mooring and riser systems is necessitated by the desire to reduce the TLP displacement and minimize the need for complicated tensioning and handling systems. The mooring and riser systems are subjected to more than 100,000,000 loading cycles during a common service life for a tension leg platform. This makes corrosion and corrosion fatigue resistance an important design parameter. Therefore, the selection of a corrosion protection system that achieves long term corrosion protection and minimizes the influence of the seawater environment on fatigue resistance is essential to insure the integrity of the high strength steel components.
The most common approach to corrosion protection involves the use of aluminum anodes. Such a system suffers the disadvantage that the cathodic potential on the steel with respect to such aluminum anodes approaches minus 1,050 mV verses a saturated calomel electrode (SCE). This cathodic level can result in hydrogen embrittlement in the high strength steel used in mooring elements. Testing has shown that a cathodic potential below negative 800 mV (SCE) subjects the high strength steel to hydrogen embrittlement thereby limiting the cracking resistance and fatigue life of the mooring and riser systems. Additionally, a reliable electrical contact must be maintained between a sacrificial anode and the high strength steel tubulars. The electrical attachment method must not impair the mechanical or the metalurgical performance of the steel. Mechanical electrical connections are generally not reliable and not recommended for long term use. Brazing and thermit welding can enhance the potential of stress corrosion cracking of high strength steels. Friction welding of an aluminum stud to the high strength steel has also been shown to cause failure in test specimens when cracks initiated either under the stud or at the edge of the weld.
An impressed current system for this application would involve throwing current from anodes located on the hull of the floating structure. The distance between anodes and remote components would be too great for effective control of the impressed current particularly at remote locations such as the anchor end of the mooring system.
For the protection of high strength steel components such as the mooring and riser systems for TLPs, the use of inert coatings cannot be seriously considered without the addition of cathodic protection because of the inevitable damage to and water permeation of the coatings through the life of the platform. Also, some areas of the components have tolerances that do not permit coating. With coatings, the size of the required sacrificial anodes would be greatly reduced but the electrical connection and hydrogen embrittlement problems would still be present.